Why Cumulus Clouds Rarely Form In Summer Over Lake Okeechobee During Daylight Hours?

Answer 1

Because water has a higher heat capacity than land, the air over land becomes convective sooner than the air over water. (Convection is the upward movement of air, similar to what happens in a pot of water that has just been placed over a burner.)

Another way of saying this is that the air immediately above the lake rises in temperature more slowly than the air immediately above the surrounding land. As a result, the air over the land forms thermals sooner, which eventually forms clouds. This process of cloud formation over land actually tends to inhibit formation of clouds over water, and that's why you notice how rare clouds are over Okeechobee.

Of course, if it's sufficiently moist or if there's a weather system passing through (like a front), you'll get those clouds over the lake. But in summer, that's generally uncommon.